Collapse Of Gas Research Articles

EFFECTIVE MODELS FOR STATISTICAL STUDIES OF GALAXY-SCALE GRAVITATIONAL LENSING

We have worked out simple analytical formulae that accurately approximate the relationship between the position of the source with respect to the lens center and the amplification of the images, hence the lens cross section, for realistic lens profiles. We find that, for essentially the full range of parameters either observationally determined or yielded by numerical simulations, the combination of dark matter and star distribution can be very well described, for lens radii relevant to strong lensing, by a simple power-law whose slope is very weakly dependent on the parameters characterizing the global matter surface density profile and close to isothermal in agreement with direct estimates for individual lens galaxies. Our simple treatment allows an easy insight into the role of the different ingredients that determine the lens cross section and the distribution of gravitational amplifications. They also ease the reconstruction of the lens mass distribution from the observed images and, vice-versa, allow a fast application of ray-tracing techniques to model the effect of lensing on a variety of source structures. The maximum amplification depends primarily on the source size. Amplifications larger than ~20 are indicative of compact source sizes at high-z, in agreement with expectations if galaxies formed most of their stars during the dissipative collapse of cold gas. Our formalism has allowed us to reproduce the counts of strongly lensed galaxies found in the H-ATLAS SDP field. While our analysis is focussed on spherical lenses, we also discuss the effect of ellipticity and the case of late-type lenses (showing why they are much less common, even though late-type galaxies are more numerous). Furthermore we discuss the effect of a cluster halo surrounding the early-type lens and of a supermassive black hole at its center.

Collapse of a Bose gas: Kinetic approach

We have analytically explored temperature dependence of critical number of particles for the collapse of a harmonically trapped attractively interacting Bose gas below the condensation point by introducing a kinetic approach within the Hartree-Fock approximation. The temperature dependence obtained by this easy approach is consisted with that obtained from the scaling theory.

Radiative feedback and cosmic molecular gas: numerical method

We present results from self-consistent 3D numerical simulations of cosmic structure formation with a multi-frequency radiative transfer scheme and non-equilibrium molecular chemistry of 13 primordial species (e-, H, H+, H-, He, He+, He++, H2, H2+, D, D+, HD, HeH+), performed by using the simulation code GADGET. We describe our implementation and show tests for ionized sphere expansion in a static and dynamic density field around a central radiative source, and for cosmological abundance evolution coupled with the cosmic microwave background radiation. As a demonstrative application of radiative feedback on molecular gas, we run also cosmological simulations of early structure formation in a ~1Mpc size box. Our tests agree well with analytical and numerical expectations. Consistently with other works, we find that ionization fronts from central sources can boost H2 fractions in shock-compressed gas. The tight dependence on H2 lead to a corresponding boost of HD fractions, as well. We see a strong lowering of the the typical molecular abundances up to several orders of magnitudes which partially hinders further gas collapse of pristine neutral gas, and clearly suggests the need of re-ionized gas or metal cooling for the formation of the following generation of structures.

Supermassive black hole ancestors

We study a model in which supermassive black holes (SMBHs) can grow by the combined action of gas accretion on heavy seeds and mergers of both heavy (m_s^h=10^5 Msol) and light (m_s^l = 10^2 Msol) seeds. The former result from the direct collapse of gas in T_s^h >1.5x10^4K, H_2-free halos; the latter are the endproduct of a standard H_2-based star formation process. The H_2-free condition is attained by exposing halos to a strong (J_21 > 10^3) Lyman-Werner UV background produced by both accreting BHs and stars, thus establishing a self-regulated growth regime. We find that this condition is met already at z close to 18 in the highly biased regions in which quasars are born. The key parameter allowing the formation of SMBHs by z=6-7 is the fraction of halos that can form heavy seeds: the minimum requirement is that f_heavy>0.001; SMBH as large as 2x10^10 Msol can be obtained when f_heavy approaches unity. Independently of f_heavy, the model produces a high-z stellar bulge-black hole mass relation which is steeper than the local one, implying that SMBHs formed before their bulge was in place. The formation of heavy seeds, allowed by the Lyman-Werner radiative feedback in the quasar-forming environment, is crucial to achieve a fast growth of the SMBH by merger events in the early phases of its evolution, i.e. z>7. The UV photon production is largely dominated by stars in galaxies, i.e. black hole accretion radiation is sub-dominant. Interestingly, we find that the final mass of light BHs and of the SMBH in the quasar is roughly equal by z=6; by the same time only 19% of the initial baryon content has been converted into stars. The SMBH growth is dominated at all epochs z > 7.2 by mergers (exceeding accretion by a factor 2-50); at later times accretion becomes by far the most important growth channel. We finally discuss possible shortcomings of the model.

The structure of the interstellar medium of star-forming galaxies

We present numerical methods for including stellar feedback in galaxy-scale simulations. We include heating by SNe (I & II), gas recycling and shock-heating from O-star & AGB winds, HII photoionization, and radiation pressure from stellar photons. The energetics and time-dependence are taken directly from stellar evolution models. We implement these in simulations with pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to massive z~2 starburst disks. Absent feedback, gas cools and collapses without limit. With feedback, the ISM reaches a multi-phase steady state in which GMCs continuously form, disperse, and re-form. Our primary results include: (1) Star forming galaxies generically self-regulate at Toomre Q~1. Most of the volume is in diffuse hot gas with most of the mass in dense GMC complexes. The phase structure and gas mass at high densities are much more sensitive probes of stellar feedback physics than integrated quantities (Toomre Q or gas velocity dispersion). (2) Different feedback mechanisms act on different scales: radiation & HII pressure are critical to prevent runaway collapse of dense gas in GMCs. SNe and stellar winds dominate the dynamics of volume-filling hot gas; however this primarily vents out of the disk. (3) The galaxy-averaged SFR is determined by feedback. For given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect; but changing feedback mechanisms directly translates to shifts off the Kennicutt-Schmidt relation. (4) Self-gravity leads to marginally-bound GMCs with an ~M^-2 mass function with a cutoff at the Jeans mass; they live a few dynamical times before being disrupted by stellar feedback and turn ~1-10% of their mass into stars (increasing from dwarfs through starburst galaxies). Low-mass GMCs are preferentially unbound.

Degradation of carbamazepine in environmentally relevant concentrations in water by Hydrodynamic-Acoustic-Cavitation (HAC)

The antiepileptic drug carbamazepine is one of the most abundant pharmaceuticals in the German aquatic environment. The effect of low carbamazepine concentrations (1–50μgL−1) is discussed controversially, but ecotoxicological studies revealed reproduction toxicity, decreased enzymatic activity and bioaccumulation in different test organisms. Therefore, as a preventive step, an efficient and cost-effective technique for wastewater treatment plants is needed to stop the entry of pharmaceuticals into the aquatic environment. Cavitation, the formation, growth, and subsequent collapse of gas- or vapor-filled bubbles in fluids, was applied to solve this problem. The technique of Hydrodynamic-Acoustic-Cavitation was used showing high synergistic effect. Under optimized conditions carbamazepine (5μgL−1) was transformed by pseudo-first order kinetics to an extent of >96% within 15min (27% by hydrodynamic cavitation, 33% by acoustic cavitation). A synergistic effect of 63% based on the sum of the single methods was calculated. Carbamazepine concentrations were monitored by a sensitive and selective immunoassay and after 60min no known metabolites were detectable by LC-MS/MS.

Low-mass black holes as the remnants of primordial black hole formation

Bridging the gap between the approximately ten solar mass 'stellar mass' black holes and the 'supermassive' black holes of millions to billions of solar masses are the elusive 'intermediate-mass' black holes. Their discovery is key to understanding whether supermassive black holes can grow from stellar-mass black holes or whether a more exotic process accelerated their growth soon after the Big Bang. Currently, tentative evidence suggests that the progenitors of supermassive black holes were formed as ∼10(4)-10(5) M(⊙) black holes via the direct collapse of gas. Ongoing searches for intermediate-mass black holes at galaxy centres will help shed light on this formation mechanism.

The Role of Gravitational Instabilities in the Feeding of Supermassive Black Holes

I review the recent progresses that have been obtained, especially through the use of high-resolution numerical simulations, on the dynamics of self-gravitating accretion discs. A coherent picture is emerging, where the disc dynamics is controlled by a small number of parameters that determine whether the disc is stable or unstable, whether the instability saturates in a self-regulated state or runs away into fragmentation, and whether the dynamics is local or global. I then apply these concepts to the case of AGN discs, discussing the implications of such evolution on the feeding of supermassive black holes. Nonfragmenting, self-gravitating discs appear to play a fundamental role in the process of formation of massive black hole seeds at high redshift ( 10–15) through direct gas collapse. On the other hand, the different cooling properties of the interstellar gas at low redshifts determine a radically different behaviour for the outskirts of the accretion discs feeding typical AGNs. Here the situation is much less clear from a theoretical point of view, and while several observational clues point to the important role of massive discs at a distance of roughly a parsec from their central black hole, their dynamics is still under debate.

Goldilocks Black Holes

Astronomers are investigating middleweight, or intermediate-mass, black holes to understand how black holes found at the center of large galaxies from which nothing, not even light, can escape formed so rapidly. The research suggests that middleweight black holes formed by the collapse of primordial gas clouds in the early universe that bypassed the star stage rather than early stars providing small seeds that merged together. Additional clues about middleweight holes may come from studying bulgeless galaxies which conceal middleweight holes behind dust. INSETS: What Were the Seeds of Supermassive Black Holes?;Early Evidence Favors Gas Collapse over Star Mergers.

Observation of trapped gas during electrical resistance heating of trichloroethylene under passive venting conditions

A two-dimensional experiment employing a heterogeneous sand pack incorporating two pools of trichloroethylene (TCE) was performed to assess the efficacy of electrical resistance heating (ERH) under passive venting conditions. Temperature monitoring displayed the existence of a TCE-water co-boiling plateau at 73.4 °C, followed by continued heating to 100 °C. A 5 cm thick gas accumulation formed beneath a fine-grained capillary barrier during and after co-boiling. The capillary barrier did not desaturate during the course of the experiment; the only pathway for gas escape being through perforated wells traversing the barrier. The thickness of the accumulation was dictated by the entry pressure of the perforated well. The theoretical maximum TCE soil concentration within the region of gas accumulation, following gas collapse, was estimated to be 888 mg/kg. Post-heating soil sampling revealed TCE concentrations in this region ranging from 27 mg/kg to 96.7 mg/kg, indicating removal of aqueous and gas phase TCE following co-boiling as a result of subsequent boiling of water. The equilibrium concentrations of TCE in water corresponding to the range of post-treatment concentrations in soil (6.11 mg/kg to 136 mg/kg) are calculated to range from 19.8 mg/l to 440 mg/l. The results of this experiment illustrate the importance of providing gas phase venting during the application of ERH in heterogeneous porous media.

The age of the Milky Way halo stars from the Sloan Digital Sky Survey

We determined the age of the stellar content of the Galactic halo by considering main-sequence turn-off stars. From the large number of halo stars provided by Sloan Digital Sky Survey, we could accurately detect the turn-off as a function of metallicity of the youngest dominant population, which was done by looking at the hottest (bluest) stars of a population. Using the turn-off temperature of a population of a given metallicity, we looked for the isochrones with that turn-off temperature and metallicity and found no age gradient as a function of metallicity. This would mean that this dominating population of the Galactic halo formed rapidly, probably during the collapse of the proto-Galactic gas. Moreover, we could find a significant number of stars with hotter temperatures than the turn-off, which might be blue horizontal branch (BHB) stars, blue stragglers, or main sequence stars that are younger than the dominant population and were probably formed in external galaxies and accreted later on to our Milky Way. Motivated by the current debate about the efficiency of gravitational settling (atomic diffusion) in the interior of old solar-type stars, we used isochrones with and without settling to determine the ages. When ignoring diffusion in the isochrones we obtained ages of 14−16 Gyr. This result is a strong argument against inhibited diffusion in old halo field stars, since it results in a serious conflict with the age of the Universe of 13.7 Gyr. The age obtained including diffusion in the isochrones was 10−12 Gyr, which agrees with the absolute age of the old globular clusters in the inner halo.

Supermassive black holes and galaxy kinematics

The statistical relation between the masses of supermassive black holes (SMBHs) in disk galaxies and the kinematic properties of their host galaxies is analyzed. Velocity estimates for several galaxies obtained earlier at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and the data for other galaxies taken from the literature are used. The SMBH masses correlate well with the rotational velocities at a distance of R ≈ 1 kpc, V1, which characterize the mean density of the central region of the galaxy. The SMBH masses correlate appreciably weaker with the asymptotic velocity at large distances from the center and the angular velocity at the optical radius R25. We have found for the first time a correlation between the SMBH mass and the total mass of the galaxy within the optical radius R25, M25, which includes both baryonic and “dark” mass. The masses of the nuclear star clusters in disk galaxies (based on the catalog of Seth et al.) are also related to the dynamical mass M25; the correlations with the luminosity and rotational velocity of the disk are appreciably weaker. For a given value of M25, the masses of the central cluster are, on average, an order of magnitude higher in S0-Sbc galaxies than in late-type galaxies, or than the SMBH masses. We suggest that the growth of the SMBH occurs in the forming “classical” bulge of the galaxy over a time < 109 yr, during a monolithic collapse of gas in the central region of the protogalaxy. The central star clusters form on a different time scale, and their stellar masses continue to grow for a long time after the growth of the central black hole has ceased, if this process is not hindered by activity of the nucleus.

The Chemistry of the Early Universe

AbstractThe chemistry of the early Universe is a fascinating field of study. Even in the absence of any elements heavier than lithium, a surprising degree of chemical complexity proves to be possible, giving the topic considerable interest in its own right. In addition, the fact that molecular hydrogen plays a key role in the formation of the first stars and galaxies means that if we want to understand the formation of these objects, we must first develop a good understanding of the chemical evolution of the gas. In this review, I first give a brief introduction to the chemistry occurring in the gas prior to the formation of the first stars and galaxies, and then go on to discuss in more detail the main chemical processes occurring during the gravitational collapse of gas from intergalactic to protostellar densities, and how these processes influence the final outcome of the collapse.

HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS

Disk galaxies at high redshift (z~2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the ISM as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion-dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early-type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.

Accretion on to black holes formed by direct collapse

One possible scenario for the formation of massive black holes (BHs) in the early Universe is from the direct collapse of primordial gas in atomic-cooling dark matter haloes in which the gas is unable to cool efficiently via molecular transitions. We study the formation of such BHs, as well as the accretion of gas onto these objects and the high energy radiation emitted in the accretion process, by carrying out cosmological radiation hydrodynamics simulations. In the absence of radiative feedback, we find an upper limit to the accretion rate onto the central object which forms from the initial collapse of hot (~ 10^4 K) gas of the order of 0.1 MSun per year. This is high enough for the formation of a supermassive star, the immediate precursor of a BH, with a mass of the order of 10^5 MSun. Assuming that a fraction of this mass goes into a BH, we track the subsequent accretion of gas onto the BH self-consistently with the high energy radiation emitted from the accretion disk. Using a ray-tracing algorithm to follow the propagation of ionizing radiation, we model in detail the evolution of the photoionized region which forms around the accreting BH. We find that BHs with masses of the order of 10^4 MSun initially accrete at close to the Eddington limit, but that the accretion rate drops to of order 1 percent of the Eddington limit after ~ 10^6 yr, due to the expansion of the gas near the BH in response to strong photoheating and radiation pressure. One signature of the accretion of gas onto BHs formed by direct collapse, as opposed to massive Pop III star formation, is an extremely high ratio of the luminosity emitted in He II 1640 to that emitted in H_alpha (or Ly_alpha); this could be detected by the James Webb Space Telescope. Finally, we briefly discuss implications for the coevolution of BHs and their host galaxies.

First star formation with dark matter annihilation

We include an energy term based on dark matter (DM) self-annihilation during the cooling and subsequent collapse of the metal-free gas, in haloes hosting the formation of the first stars in the Universe. We find that the feedback induced on the chemistry of the cloud does modify the properties of the gas throughout the collapse. However, the modifications are not dramatic, and the typical Jeans mass within the halo is conserved throughout the collapse, for all the DM parameters we have considered. This result implies that the presence of DM annihilations does not substantially modify the initial mass function of the first stars, with respect to the standard case in which such additional energy term is not taken into account. We find also that when the rate of energy produced by the DM annihilations and absorbed by the gas equals the chemical cooling (at densities yet far from the actual formation of a protostellar core) the structure does not halt its collapse, although that proceeds more slowly by a factor smaller than few per cent of the total collapse time.

Novae ejecta as colliding shells

Following on our initial absorption-line analysis of fifteen novae spectra we present additional evidence for the existence of two distinct components of novae ejecta having different origins. As argued in Paper I one component is the rapidly expanding gas ejected from the outer layers of the white dwarf by the outburst. The second component is pre-existing outer, more slowly expanding circumbinary gas that represents ejecta from the secondary star or accretion disk. We present measurements of the emission-line widths that show them to be significantly narrower than the broad P Cygni profiles that immediately precede them. The emission profiles of novae in the nebular phase are distinctly rectangular, i.e., strongly suggestive of emission from a relatively thin, roughly spherical shell. We thus interpret novae spectral evolution in terms of the collision between the two components of ejecta, which converts the early absorption spectrum to an emission-line spectrum within weeks of the outburst. The narrow emission widths require the outer circumbinary gas to be much more massive than the white dwarf ejecta, thereby slowing the latter's expansion upon collision. The presence of a large reservoir of circumbinary gas at the time of outburst is suggestive that novae outbursts may sometime be triggered by collapse of gas onto the white dwarf, as occurs for dwarf novae, rather than steady mass transfer through the inner Lagrangian point.

Supermassive black hole formation by direct collapse: keeping protogalactic gas H2free in dark matter haloes with virial temperaturesTvir&gt;rsim 104K

In the absence of H_2 molecules, the primordial gas in early dark matter halos with virial temperatures just above T_vir >~ 10^4 K cools by collisional excitation of atomic H. Although it cools efficiently, this gas remains relatively hot, at a temperature near T ~ 8000 K, and consequently might be able to avoid fragmentation and collapse directly into a supermassive black hole (SMBH). In order for H_2--formation and cooling to be strongly suppressed, the gas must be irradiated by a sufficiently intense ultraviolet (UV) flux. We performed a suite of three--dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic halos with T_vir >~ 10^4 K, irradiated by a UV flux with various intensities and spectra. We determined the critical specific intensity, Jcrit, required to suppress H_2 cooling in each of the three halos. For a hard spectrum representative of metal--free stars, we find (in units of 10^{-21} erg s^{-1} Hz^{-1} sr^{-1} cm^{-2}) 10^4<Jcrit<10^5, while for a softer spectrum, which is characteristic of a normal stellar population, and for which H^{-} --dissociation is important, we find 30<Jcrit<300. These values are a factor of 3--10 lower than previous estimates. We attribute the difference to the higher, more accurate H_2 collisional dissociation rate we adopted. The reduction in Jcrit exponentially increases the number of rare halos exposed to super--critical radiation. When H_2 cooling is suppressed, gas collapse starts with a delay, but it ultimately proceeds more rapidly. The infall velocity is near the increased sound speed, and an object as massive as M ~ 10^5 solar mass may form at the center of these halos, compared to the M ~ 10^2 solar mass stars forming when H_2--cooling is efficient.

Effects of SiNx on two-dimensional electron gas and current collapse of AlGaN/GaN high electron mobility transistors

SiNx is commonly used as a passivation material for AlGaN/GaN high electron mobility transistors (HEMTs). In this paper, the effects of SiNx passivation film on both two-dimensional electron gas characteristics and current collapse of AlGaN/GaN HEMTs are investigated. The SiNx films are deposited by high- and low-frequency plasma-enhanced chemical vapour deposition, and they display different strains on the AlGaN/GaN heterostructure, which can explain the experiment results.

Direct formation of supermassive black holes via multi-scale gas inflows in galaxy mergers

Observations of distant quasars indicate that supermassive black holes of billions of solar masses already existed less than a billion years after the Big Bang. Models in which the 'seeds' of such black holes form by the collapse of primordial metal-free stars cannot explain the rapid appearance of these supermassive black holes because gas accretion is not sufficiently efficient. Alternatively, these black holes may form by direct collapse of gas within isolated protogalaxies, but current models require idealized conditions, such as metal-free gas, to prevent cooling and star formation from consuming the gas reservoir. Here we report simulations showing that mergers between massive protogalaxies naturally produce the conditions for direct collapse into a supermassive black hole with no need to suppress cooling and star formation. Merger-driven gas inflows give rise to an unstable, massive nuclear gas disk of a few billion solar masses, which funnels more than 10(8) solar masses of gas to a sub-parsec-scale gas cloud in only 100,000 years. The cloud undergoes gravitational collapse, which eventually leads to the formation of a massive black hole. The black hole can subsequently grow to a billion solar masses on timescales of about 10(8) years by accreting gas from the surrounding disk.